The present invention relates generally to underwater navigation and, more particularly, to a method and apparatus for extending GPS navigation to divers and underwater vehicles.
Satellite-based positioning systems, such as the Global Positioning System (GPS), provide the ability to accurately determine location virtually almost anywhere on or above the Earth's surface. The GPS system comprises 24 earth-orbiting satellites located in 6 orbital planes. Each earth-orbiting satellite carries an atomic clock and continuously broadcasts radio signals indicating its current time and location. A receiver located on the Earth's surface can receive these radio signals and determine its distance from the satellites based on the time of arrival of the signals. By receiving signals from four satellites, an Earth-based receiver can determine its location by triangulation.
GPS signals do not propagate underwater. Consequently, divers and underwater vehicles beneath the water's surface are not able utilize these GPS signals to navigate accurately. A number of systems have been proposed for extending GPS to underwater divers and vehicles. For example, U.S. Pat. Nos. 6,701,252 to Brown and 6,657,585 to Kucik disclose a floating buoy that is connected by a tether to a diver or underwater vehicle. The floating buoy carries a GPS antennas and/or receiver and conveys signals via the tether to the diver or underwater vehicle. This solution is limited in utility by the need for a tether connecting the underwater diver or vehicle to the floating buoy. U.S. Pat. No. 5,119,341 to Youngberg discloses a system for extending GPS to divers and vehicles beneath the water's surface using buoys that float freely on the surface. The floating buoys can receive signals from GPS satellites and can communicate underwater users using acoustic signals. However, the floating buoys do not stay in place, but instead drift on the surface of the water. Further, floating buoys are subject to easy detection and thus are not suitable for covert operations.